By definition, a hypsometer is a boiling point barometer by which the atmospheric pressure can be determined by way of the relation between the saturation vapour pressure and the boiling temperature. Thereby, the boiling temperature changes logarithmically with the pressure. This has the advantage that in applications which have a wide pressure span, the relative pressure measuring accuracy of the boiling temperature. Metrological radio-sondes, while rising to a height of 35 km, traverse a pressure range of 1000 hPa to 5 hPa. Therefore, the hypsometers are more qualified than the aneroidal barometers generally used. A further essential advantage of the hypsometer as compared with the aneroidal barometer is that the former does not need an individual calibration, since the relation of pressure to boiling temperature is known with sufficient accuracy. Sondes which do not need calibration can be reused without more ado after they are located. This is reasonable for ecological reasons too. Therefore, it was proposed already some decades ago to equip radio-sondes with hypsometers.
Heated hypsometers use a fluid, e.g. water, the boiling temperature of which is higher than the ambient temperature. The heating capacity must not be high, since it is to be produced by batteries which are flying along with the sonde. Thus, a small quantity of fluid is to be aimed at. In order to achieve a sufficiently long service life with such a small quantity of fluid, it is to be avoided that during operation of the hypsometer any fluid is lost by condensation of the vapour outside the boiling vessel. On the other hand, the heating capacity is to be constantly high enough for maintaining boiling over the entire pressure range. In order to satisfy these requests, a suitable control of the heating capacity is to be provided for.
In a known hypsometer of this kind (cf. Lueger, Lexikon der Technik, 4th Edition, Stuttgart 1968, Volume 13, Page 456), a control of the heating capacity is provided by means of a thermostat having its temperature measuring point within the vapour expansion space. However, this control is only operative in that narrow range of the boiling temperature where, near the temperature measuring point of the thermostat, it can be operated at a constant command temperature. As soon as the boiling temperature is above or below said range, the temperature at the measuring point drops out of the control range of the thermostat, the heating capacity thus always remaining fully switched-on upon further lowering the boiling temperature, and remaining fully switched-off upon further raising the boiling temperature. However, in these phases an ordinary working of the hypsometer is no longer ensured, since the said marginal conditions are no longer met. Therefore, this hypsometer may be suitable for a stationary operation but is not suitable for metrological radio-sondes in which, during their flight, the boiling temperature may change from 100.degree.C. to a few degrees centigrade.
The heating capacity which is necessary for boiling decreases as the altitude increases, since on the one hand the atmospheric pressure, and therewith the boiling temperature, decrease, and on the other hand the thermal conductivity of the air, and therewith the heat loss, decrease when the atmospheric pressure decreases. Even under these conditions, the hypsometer works properly if the great heating capacity, which is necessary for the pressure on the ground, is continously supplied to it. The reason for this tolerance of the instrument is the great evaporation heat of water. However, a continuous supply of too much heating capacity is unwanted, not only for the risk of excessive loss of fluid, but also for reasons of supply. In radio-sonde applications, beside the costs, the weight of unnecessary large batteries is disturbing.